Positioning drive, in particular for a machine tool

ABSTRACT

A positioning drive having a hydraulic motor with a cam disc which is rotatable with a motor shaft and has a cam, a control valve which, with the hydraulic motor forms a unit with a housing and has a valve piston which is displaceable between several positions, a control element via which the valve piston can be supported on the cam disc, and an adjustment element which is operatively disposed between the control element and the valve piston and coaxial to the latter. The relative position between the valve piston and control element is variable by relative adjustment of the adjustment element with respect to the valve piston. The position of the adjustment element relative to the valve piston is securable by a releasable locking element. The outer of the two parts, valve piston and adjustment element, is guided outwards through the housing, and the locking element is accessible externally of the housing.

FIELD AND BACKGROUND OF THE INVENTION

The present invention concerns from a positioning drive which is used,in particular, on machine tools and which has the features set forth inthe preamble of the independent claim.

Such a positioning drive is used, in particular, in order to bring amachine part for which several different positions are preferablypossible, into a given position. The machine part can, for instance, bea multi-spindle turret of a boring machine or a tool holder of a machinehaving several machining stations.

A positioning drive having the features introductory-mentioned is knownfrom the Japanese utility model application published under 62/195435.In that positioning drive, the housing part of the control valve isfastened to the housing part of the hydraulic motor in such a mannerthat the axis of the valve piston extends perpendicular and tangentialto the axis of the motor shaft. The control element is a double-armlever which is so mounted in the motor housing that it is swingablearound an axis which extends parallel to the motor shaft. At the end ofthe one lever arm, the control element can act on the cam on the camdisk which is fastened, fixed for rotation, on the motor shaft, thelever arm extending approximately perpendicular to a radius vectorbetween the axis of the motor shaft and the place of contact between camdisk and control element. At the end of the other lever arm, the controlelement grips in fork-shaped manner around a setting element which isfastened to the valve piston. The setting element is developed as abushing which has, on its outer side, two driver pins which are locateddiametrically opposite each other and which are gripped around by thecontrol element, the bushing being screwed by an internal thread on asection of the valve piston which is provided with an external thread.The relative position between valve piston and setting element issecured by means of a lock nut.

Another positioning drive is known from Federal Republic of Germany AS23 49 099. In that positioning drive, the control element is a controlbolt which is developed in one piece with the valve. Control bolt andvalve piston are movable radially with respect to the cam disk. Finally,in the senior German Patent Application P 43 12 581.6 a positioningdrive is described in which a control piston and a valve piston arearranged, also aligned with each other, radially to the cam disk.Control piston and valve piston are to be sure two separate parts.

In the positioning drives indicated, the valve piston can be displacedhydraulically in both directions of movement. By movement in onedirection, the control element is lifted off from the cam. In thisposition of the control element and of the valve piston the motor turnswith the maximum speed provided. Movement of the valve piston in theother direction is possible only until the control element comes againstthe cam and is then determined by the shape of the cam as long as thevalve piston is subjected to a force acting in the correspondingdirection. If the control element lies against the cam in a region inwhich the latter is at a constant distance from the axis of the motorshaft then the valve piston remains at rest. In a region in which thedistance of the cam from the axis of the motor shaft is reduced, thevalve piston moves in the direction of the force acting on it. Inpositions of the valve piston in which the latter rests via the controlelement against the cam, a throttle is connected on the discharge sidein the hydraulic circuit, the throttling action of which is dependentwithin a limited region on the position of the valve piston. The motornow moves at creep feed in order to reach the selected position. Whenthe selected position is reached, the control element falls into anindentation in the cam which is associated one or more positions of themachine part, and the valve piston enters into a position in which thehydraulic motor is no longer supplied with pressurized fluid.

In the positioning drive known from Federal Republic of Germany AS 23 49099 and in the positioning drive described in the senior German PatentApplication P 43 12 581.6, the action of the throttle cannot beadjusted. In the positioning drive known from the Japanese utility modelapplication, the lock nut screwed onto the section of the valve pistonprovided with an external thread can be loosened and the valve pistonand the setting element displaced axially with respect to each other byrotating these two parts with respect to each other. In this way, therelative position is changed between the valve piston and the controlelement so that the action of the throttle is changed for a givenposition of the control element. In this way, the drive can be adaptedto different circumstances upon different cases of use. In the case ofthe known positioning drive, the lock nut is located within a cavity inthe housing into which the valve piston extends, which is filled withhydraulic oil upon operation and has, in the extension of the valvepiston, an opening which is closed by a closure screw. In order toloosen the lock nut, it is therefore necessary to remove the closurescrew. Upon removal of the closure screw, oil flows out of the cavityunless the oil level has been previously lowered. Finally, after anadjustment and after the closure screw has been screwed on, the motormust be vented.

SUMMARY OF THE INVENTION

The object of the present invention is so to improve a positioning driveof the introductory-mentioned type wherein the adjustment of the valvepiston with respect to the control element can be effected in a simplemanner in substantially less time.

This object is achieved in such a positioning drive in the manner thatthe outer one of the two parts which are coaxial to each other, namelythe valve piston or the setting element, is guided outward through thehousing of the structural unit comprising the control valve and thehydraulic motor, and that the securing element, for instance a lock nut,is accessible from outside the housing. Accessible from outside thehousing does not mean, in this connection, that the securing element islocated outside of the outer contour of the housing. It merely meansthat it is not necessary to open the housing in order to reach thesecuring element. There is conceivable in this connection, for instance,a construction in which the securing element is located within the outerone of the two parts, valve piston and setting element, and within theouter contour of the housing but that it is readily accessible from theoutside, for instance through the outwardly extended bar which isprovided with an axial passage.

As already indicated, it is basically sufficient if the outer one of thetwo parts, valve piston and setting member, is conducted outward out ofthe housing. In the case of a construction in accordance with theJapanese utility model application, this would be the bushing-likesetting member. The length of the valve piston need not be changed. Assecuring element there can be used, for instance, a lock screw which isscrewed so far from the outside into the bushing-like setting elementthat it lies tightly against the valve piston. In this connection, ashort locking screw which does not extend beyond the setting element andwhich is provided with an internal socket or a slot in order for a toolto act on it is sufficient. The securing element is, however, moreeasily accessible if a positioning drive in accordance with theinvention is further developed, in addition to the outer of the twoparts the inner of the two parts, valve piston and setting element, isalso conducted outward, the inner part protruding beyond the outer partand the securing element being a threaded nut threaded on the innerpart.

In order that no oil can leak out in the region of the housing in whichthe valve piston and/or the setting element are guided out of thehousing, the positioning drive is sealed in a manner.

If the control element is arranged on a first end of the valve piston,then an adjustment is possible from the other end of the valve piston bya development wherein, the second end of the valve piston extends out ofthe structural unit and has a continuous axial bore. The adjustmentelement is contained, as adjustment rod, in the axial bore of the valvepiston. Furthermore, a screw joint between adjustment element and valvepiston is formed by an external thread on the adjustment rod and aninternal thread on the valve piston. By turning the valve piston withrespect to the adjustment rod, the relative position between valvepiston and control element is changed. The adjustment rod advantageouslyextends by an externally threaded section by which it engages beyond thevalve piston into the internal thread of the valve piston. A lock nut isthen screwed on this externally threaded section in order to secure theposition of adjustment rod and valve piston with respect to each other.Therefore, there are not present separate externally threaded sectionson the adjustment rod for the valve piston and the lock nut.

A development is particularly advantageous when the control element isarranged as a control bolt in the extension of the valve piston. Theadjustment rod is advantageously also developed then as a separatestructural part and fastened on the control bolt, in particular screwedto the control bolt. If the control bolt, which is arranged in theextension of the valve piston, is guided, secured against rotation, inthe housing, no separate direct anti-turning means is necessary betweenthe adjustment rod and the housing if the adjustment rod, in accordancewith claim 7, is connected in a manner secured against rotation to thecontrol bolt.

In accordance with a feature of the invention, two fluid connections ofthe structural unit can be connected together via the axial bore of thevalve piston. The axial bore therefore fulfills a two-fold function. Onthe one hand, it receives the adjustment rod and, on the other hand, itis part of a possible fluid communication between two connections. Inthis way, it may be possible to dispense, as compared with a known orpreviously described positioning drive, with one or more housing boresor to enlarge the cross section of passage between the two fluidconnections of the structural unit. Since the dispensing with housingbores or the enlarging of the cross section of flow through an axialbore in the valve piston is also possible when the valve piston is notadjustable with respect to the control element, the feature set forth inthe first sentence of this paragraph can be used to advantage also in apositioning drive which does not have the previously-mentioned features.The use of the axial bore as part of a communication between two fluidconnections is also particularly favorable especially when the axialbore is open axially to a chamber of the housing which is connected witha first fluid connection. In such case, namely, only a single furthertransverse bore in the valve piston is necessary in order to be able touse the axial bore as part of the communication between two fluidconnections.

BRIEF DESCRIPTION OF THE DRAWINGS

Two illustrative embodiments of a positioning drive in accordance withthe invention are shown in the drawings. The invention will now beexplained in further detail with reference to the figures of thedrawing, in which:

FIG. 1 diagrammatically shows the first embodiment;

FIG. 2 is a section through the first embodiment perpendicular to themotor shaft and in the axis of the valve piston;

FIG. 3 is a partial section through the first embodiment in the regionof the control piston and perpendicular to the sectional view of FIG. 2;

FIG. 4 is a view of the adjustment element of FIGS. 2 and 3, developedas adjustment rod;

FIG. 5 shows the lock nut, visible in FIGS. 2 and 3, between valvepiston and adjustment element;

FIG. 6 shows, on a larger scale, a section through the valve pistonalong the line VI--VI of FIG. 2; and

FIG. 7 shows a portion of the valve piston of a second embodiment havinga cap nut as lock nut between adjustment rod and valve piston.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The positioning drive of FIGS. 1 to 5 comprises a hydraulic motor 10,which produces a high torque at low speed, a direction of rotation valve11, a control valve 12, and a 3/2-way starting valve 13. Thedirection-of-rotation valve 11 is a 4/3-way valve, the valve piston 14of which is spring-centered and can be moved electromagnetically. In thecentral position of rest of the valve piston 14, the outlets C and D ofthe direction-of-rotation valve 11 which are connected to the hydraulicmotor 10 are separated from two inlets A and E. In the two workingpositions, the inlets are connected alternately with the outlets. Thecontrol valve 12, which has a valve piston 15, has an inlet T which isconnected with a pressurized-fluid storage container and an inlet Pwhich is connected with the delivery side of a hydraulic pump. Twooutlets of the control valve 12 correspond to the two inlets A and E ofthe direction-of-rotation valve; an outlet designated B of the controlvalve 12 is an external connection of the positioning drive. The outletA is also guided as external connection on the outside of thepositioning drive.

In the connection between the one outlet of the control valve 12 and theinlet E of the direction-of-rotation valve 11 a displaceable throttle 21is arranged by which a maximum speed of the hydraulic motor 10 can beset.

The longitudinal axis of a control bolt 16 coincides with thelongitudinal axis of the valve piston 15. Valve piston 15 and controlbolt 16 are together arranged and movable radial to a cam disk 17, thecam disk being seated, fixed for rotation, on the motor shaft 18 of thehydraulic motor 10. The control bolt 16 cooperates with a cam 19 formedby a corresponding shape of the outer circumference of the cam disk 17,the cam having two diametrically opposite detent notches 20 and the sameconstant distance from the axis of the motor shaft in the two regionsbetween the two detent notches.

The control valve 12 has a control chamber 25 which the valve piston 15divides by an outer collar 26 into two annular chambers 27 and 28 ofdifferently larger annular surfaces. The smaller annular surface of theouter collar 26, and thus the smaller annular space 27, is located, seenfrom the control block 16, on this side of the outer collar 26 and thelarger annular surface, and thus the larger annular space 28, on theother side of the outer collar 26. The annular space 27 is continuouslyconnected to the inlet P of the control valve 12. The annular space 28is connected to the starting valve 13 and in the rest position of thelatter is connected to the pressure inlet P while in the workingposition thereof it can be relieved via the inlet T of the control valve12 to the pressurized-fluid storage container. Within the annular space28 there is furthermore arranged a coil compression spring 29 whichurges the valve piston and the control bolt 16 in the direction towardsthe cam 19.

In the position shown in FIG. 1, the control bolt 16 engages, under theaction of the spring 29, into a detent notch 20 in the cam 19. Ifpressure is present at the inlet P, then the same pressure to be sureprevails in the annular spaces 27 and 28. Due to the difference in sizeof the annular surfaces, the control bolt, however, is urged into thedetent notch not only by the spring force 29 but also by a hydraulicforce. The inlet P of the control valve 12 is connected to the outlet Band the inlet T is connected to the outlet A. When pressure is presentin the inlet P, a hydraulic cylinder connected to the outlets A and Bcan therefore be actuated. In order to permit the hydraulic motor 10 tooperate, the starting valve 13 is first of all connected. As a result,the annular space 28 is relieved of load while the pressure in line Pprevails in the annular space 27. The force produced by this pressure onthe annular surface of the outer collar 26 facing the annular chamber 27overcomes the force of the spring 29 and pushes the valve piston 15radially outward with respect to the cam disk 17 until it strikes a stopwhich is so arranged that the control bolt 16 is now spaced from the camdisk 17. In this position of the valve piston 15, the outlet B of thecontrol valve 12 is connected, unthrottled, with the inlet T. If onedisregards the throttle 21, the outlet E is also connected unthrottledwith the inlet T. The outlet A is connected with the inlet P. If thedirection-of-rotation valve 11 is now switched, the hydraulic motor 10begins to turn. Shortly before a further detent notch 20 of the cam disk17, selected in accordance with the position of the machine part to beestablished, comes in front of the control bolt 16, the starting valveswitches back into its position of rest so that the annular space 28 isacted on by the same pressure as the annular space 27. Due to the largerannular surface of the annular space 28, the control bolt 16 and thevalve piston 15 are moved radially inwards towards the cam disk 17 untilthe control bolt 16 comes against a section of the cam 19 which ispresent between two detent notches. In this position, the same outletsand inlets of the of the control valve 12 are connected to each other asin the previously described position of the valve piston 15. A throttleplace 30 is merely connected between the inlet T and the outlet E andtherefore in the discharge of the hydraulic oil. The hydraulic motor 10now turns further with slower speed until the control bolt 16 can snapinto the next detent notch 20 and both motor lines are connected to thetank.

In FIG. 2 there can be noted the motor shaft 18 which, in a manner notshown, is rotatably supported on two sections spaced from each other byin each case a plain bearing within a multipartite motor housing 37.

The motor shaft 18 extends into a chamber 42 of the motor housing 37which is closed by a cover 43 and in which the cam disk 17, which isseated fixed for rotation on the motor shaft 18, is located, which camdisk has the cam 19 with the two detent notches 20 already shown inFIG. 1. In the region of the cam disk 17, a bore extending radiallythrough the motor housing 37 debouches into the chamber 42, the borecontinuing outside of the motor housing in a valve bore 45 which islocated in a valve housing 46 of the control valve 12 built onto themotor housing 37 and the axis of which coincides with the axis of thebore 44. This axis extends radially to the axis of the motor shaft 18and of the cam disk 17.

The control bolt 16 is guided in the bore 44 and the valve piston 15 isguided in the valve bore 45, the control bolt 16 and the valve piston 15being produced as separate parts. The end 48 of the control bolt whichfaces the cam disk 17 is made of wedge shape, the vertex line of thewedge extending parallel to the motor shaft. In order that the vertexline always retains this direction, the control bolt 16 is securedagainst rotation. For this purpose, the bore 44 is provided with alongitudinal groove 49 into which there engages a ball 50 which iscontained within a recess 51 in the control bolt 16. Diametricallyopposite the longitudinal groove 49 and in a plane passing through theaxis of the control bolt 16 and the axis of the cam disk 17, a secondlongitudinal groove 52 extends along the bore 44, whereby an opencommunication is produced between the chamber 42 and a valve chamber 53immediately adjacent the motor housing 37, within the valve bore 45.This valve chamber 53 is connected with the inlet T of the control valve12. The control space 25 is located, seen from the valve chamber 53, onthe other side of a narrower section of the valve bore 45 within thehousing 54 of the control valve 12 and is closed towards the outside bya cover 55 which is placed on the valve housing 46. Within the controlspace there can be noted the compression spring 29 and the outer collar26 of the valve piston 15.

The valve piston 15 is guided outward through the cover 55 and isprovided, outside the valve housing 46 to which the cover 55 alsobelongs, with a dihedral 54. The passage is sealed by a packing ring 61which is arranged in a pocket in the cover 55 between the latter andvalve piston 15.

The connections P, T, A, B and E on the valve housing 46 of the controlvalve 12 are so bored through with the valve bore 45 that theconnections discussed when explaining FIG. 1 can be produced with thevalve in the individual switched positions of the valve piston 15. FromFIG. 2 it can also be noted, in particular, that the valve chamber 53 isconnected with the connection T and that the annular space 27 isconnected continuously with the connection P via two bore holes 56.

In the region of the transition from the valve chamber 53 to thenarrower section of the valve bore 45, the valve piston 15 has twoannular collars 57 and 58 which directly adjoin each other. The innerannular collar 58 has a diameter which corresponds to the diameter ofthe narrower section of the valve bore 45. The annular collar 57 isslightly smaller in diameter than the annular collar 58. It ends with abevel 60 at the end 59 facing away from the annular collar 58.

The control bolt 16 and the valve piston 15 are rigidly attached to eachother by an adjustment rod 65 which is provided on each of its two endswith a threaded section 66 and 67 respectively. By the threaded section66 it is screwed into a central, axial threaded bore 68 made in thecontrol bolt 16 from the end facing the valve piston 15. The connectionbetween the adjustment rod 65 and the control bolt 16 is secured by alock nut 69.

Centrally through the entire valve piston 15 there extends an axial bore70 through which the adjustment rod 65 extends from the control bolt 16to the other side of the valve piston 15, so that it protrudes outsidethe valve housing 46 over the valve piston 15. Its threaded section 67is located in part within the valve piston 15 and engages there into aninternal thread 71 of the valve piston 15. On the protruding part of thethreaded section 67 of the adjustment rod 65 there is threaded a locknut 72 which rests axially firmly against the valve piston 15 and locksthe adjustment rod 65 and the valve piston 15 in their position withrespect to each other. The lock nut 72 is a collar nut in which apacking ring 73 is arranged, as can be noted more clearly from FIG. 5.Upon the tightening of the lock nut, this packing ring presses againstthe adjustment rod 65 and the valve piston 15 so that no hydraulic oilcan pass outwards via the axial bore 70 of the valve piston 15 betweenthe interengaging threaded sections 67 and 71.

The throttle point 30 between the connections T and E of the controlvalve 12 is formed in the manner that the bore 61 in the valve housing46 is not entirely covered by the annular collar 57, and therefore theend 59 of the annular collar 57 is located axially within the bore 61when the control bolt rests on the cam 19 outside of the detent notch70. The size of the surface in which the annular collar 57 leaves thebolt 61 free can now be changed by changing the distance between thecontrol bore 16 and the valve piston 15. For this purpose, the lock nut72, which is readily accessible outside of the housing, is loosened.Thereupon, a tool is placed on the dihedral 54 of the valve piston 15and the valve piston turned with respect to the adjustment rod 65. Thelatter does not also turn since, on the one hand, it is connected inunturnable manner with the control bolt 16 and since, on the other hand,the control bolt 16 is secured against turning with respect to the motorhousing 37 by the balls 50. By the turning of the valve piston 15 withrespect to the adjustment rod 65, the distance between the control bolt16 and the valve piston 15 is changed, and thus the size of the openingcross section of the throttle 30. Upon the movement of the control bolt16 into a detent notch 20, the end 59 of the valve piston 15 passes, ascan be noted from FIG. 2, out of the region of the bore 61 so that nowonly the annular slot between the annular collar 27 and the bore 45determines the throttle cross section. This cross section is independentof the previous adjustment. The bevel 60 smooths the transition betweendifferent throttle cross sections. It can be seen that, with apositioning drive in accordance with the invention, the opening crosssection of the throttle 30 which is active when the control bolt 16 lieson the cam 19 outside of a detent notch 20 can be adjusted very easily.No intervention in the valve housing 46 is necessary. No oil leaks out.Venting is avoided.

In the embodiment of FIG. 7, in which there can be noted the valvepiston 15 of a control valve 12 and the adjustment rod 65 with itsthreaded section 67, a cap nut 80 is used to fasten the valve piston 15and the adjustment rod 65 to each other instead of the collar nut 47 ofthe embodiment shown in FIGS. 2 to 6. The cap nut 80 is screwed onto thethreaded section 67 of the adjustment rod 65 and clamps a packing ring81 axially between itself and the valve piston 15. Due to the use of acap nut, this packing ring need not also rest against the adjustment rod65. Furthermore, the adjustment of the valve piston 15 with respect to acontrol piston is effected, in the embodiment of FIG. 7, in the same wayas in the embodiment of FIGS. 2 to 6. The cap nut 80 is first of allloosened and then, by action on the dihedral 54 of the valve piston 15,the latter is turned with respect to the adjustment rod 65. The cap nut80 is then again tightened.

In both embodiments, the axial bore 70 in the valve piston 15 can at thesame time be used in order to produce, via the axial bore, communicationbetween two connections of the control valve 12. For this purpose, thevalve piston 15 is furthermore provided with two transverse bores 85which are diametrically opposite each other and are present in a pistonneck 86 which is limited on the one side by a piston collar 87 whichpermanently separates the annular space 27 from the tank connection Tand, on the other side, by a piston collar 88 which controls theconnection of the connection A with the pressure connection P and thetank connection T and the connection of the connection B with thepressure connection P. Via the transverse bore 85 and the section of theaxial bore 70 present between the transverse bore 85 and the end 59, theconnection A can be connected to the tank connection T. For thispurpose, the axial bore 70 has a larger diameter in said section than inthe section between the transverse bore 85 and the end protruding fromthe housing 46.

We claim:
 1. A positioning drive, comprising a hydraulic motor (10)which has a cam disk (17) which is rotatable with a motor shaft (18) andis provided with a cam (19), a control valve (12) with which thehydraulic motor (10) forms a structural unit with a housing (37, 46) andhas a valve piston (15) which is displaceable between several positions,a control element (16) via which the valve piston (15) can rest againstthe cam disk (17), and an adjustment element (65) which is operativelyarranged between the control element (16) and the valve piston (15) andcoaxial to the valve piston (15), and wherein by relative displacementof said adjustment element with respect to the valve piston (15), therelative position between said valve piston (15) and said controlelement (16) is variable and the position of said adjustment elementwith respect to the valve piston (15) secured by a releasable lockingelement (72, 80), and wherein an outer part of the two parts, valvepiston (15) and adjustment element (65), is guided outwardly through thehousing (46), and the locking element (72, 80) is accessible fromoutside the housing (46).
 2. A positioning drive according to claim 1,wherein an inner part (65) of the two parts, valve piston (15) andadjustment element (65), is guided outwardly through the outer part, andthe inner part (65) protrudes beyond the outer part; and the securingelement is a threaded nut (72, 80) which is screwed onto the inner part(65).
 3. A positioning drive according to claim 1, wherein a firstpacking (61) is present between the housing (46) and the outward guidedsection of the outer part and the slot between the valve piston (15) andthe adjustment element (65) is sealed from the outside by a secondpacking (73,81).
 4. A positioning drive according to claim 1, whereinthe control element (16) is arranged on a first end of the valve piston(15); at its other, second end, the valve piston (15) extends out of thestructural unit (10, 12) and has a continuous axial bore (70); theadjustment element is an adjustment rod (65) and is contained within theaxial bore (70); and a threaded joint is formed between said adjustmentrod (65) and said valve piston (15) by an external thread (67) on theadjustment rod (65) and an internal thread (71) on the valve piston(15).
 5. A positioning drive according to claim 4, wherein saidexternally threaded section (67) protrudes beyond the valve piston (15),and a lock nut (72,80) is screwed on said externally threaded section(67).
 6. A positioning drive according to claim 4, wherein the controlelement is arranged as a control bolt (16) in the extension of the valvepiston (15), and the adjustment rod (65) is fastened as a separate parton the control bolt (16).
 7. A positioning drive according to claim 4,wherein the control element is arranged as a control bolt (16) in theextension of the valve piston (15) and is guided, secured againstrotation, in the housing (37), and the adjustment rod (65) is connected,secured in rotation, to the control bolt (16).
 8. A positioning drive,in particular according to claim 4, wherein two fluid connections (A, T)of the structural unit (10, 12) can be connected to each other via theaxial bore (70) of the valve piston (15).
 9. A positioning driveaccording to claim 8, wherein the axial bore (70) is open axiallytowards a chamber (53) of the housing (37, 46) which chamber isconnected with a first fluid connection (T), and the valve piston (15)has a transverse bore (85) which debouches into the axial bore (70) andis connectable to a second fluid connection (A).